16 COLIX CLOUT'S CALENDAR. 



We know that the climate of England in comparatively 

 recent times was apparently as warm as that of North 

 Africa ; and we know that at the same period the beds 

 of the Mediterranean and the English Channel were dry 

 land. Hence it was then at least as easy for the swifts 

 and swallows to range from Scotland to Sahara as it now 

 is in America for the hardier humming-birds to range 

 from Canada to Mexico. But when the change of " cos- 

 mical weather" made England by slow degrees too cold 

 in winter for flowers and midges to flourish all the year 

 round, the swallows would begin gradually to fly a little 

 to the south, as each autumn came on, and remove a lit- 

 tle to the north again as spring returned. At first, no 

 doubt, they would only have to shift their quarters very 

 slightly in search of more plentiful food, without them- 

 selves being conscious of any special migration. In 

 course of time, however, as the difference in climate be- 

 came more and more marked, the birds would have to 

 fly further and further south with each successive 

 autumn, and would be enticed further and further north 

 again to their original homes with each successive spring. 

 Thus at last the practice of migration would become 

 engrained in the nervous system, and would grow into 

 what we ordinarily call an instinct that is to say, an 

 untaught habit. This is the stage at which the migratory 

 custom has always remained in America, where broad 

 stretches of land extend from the Arctic, region to the 

 tropical forests, unbroken by any intermediate zone of 

 severing sea. 



In Europe, however, special circumstances have added 

 another and more complicated element to the problem 

 the element of discontinuity. The Mediterranean, the 

 English Channel, and the Baltic practically cut off the 

 various parts of the swallows' summer hunting-grounds 



